A series of HTLV-I cell lines were prepared by in vitro transformation of rabbit blood mononuclear cells (PBMC) using HTLV-I. Animals injected with certain infected lines (e.g.RH/K34) developed adult T cell leukemia- lymphoma-like disease (ATLL). The majority of rabbit HTLV-I T cell lines (e.g.RH/K30), including several derived in the same fashion as the lethal line, cause no overt disease. Results of a detailed study may shed light on why HTLV-I infection in humans causes serious disease in only a small percentage of infected individuals. The labortory has succeeded in defining an HTLV-I molecular clones to use for a study of specific viral genes. Several T cell and fibroblast lines transfected with the clone derived from RH/K30 expressed HTLV-I p24 gag protein at levels comparable to the donor RH/K30 line. A transfected rabbit cell line, RL-5/K30 was able to transmit HTLV-I infection as measured by both in vitro and in vivo assays. Rabbit and human lymphocytes cocultured with irradiated RL- 5/K30 cells became infected with HTLV-I. Rabbits injected with RL-5/K30 cells produced an HTLV-I antibody response, and HTLV-I was isolated from PBMC beginning 3 months after injection. In order to study HTLV-I factors involved in persistence of viral protein expression, chimeric molecular clones have been constructed by shuffling genes of K30 and K34. An initial result implies that the viral genes other than the LTR region are important for maintenance of p24 expression in transfected RL-5 cells and assignment of the precise region involved in viral expression will be addressed by substituting each of the differences between K30 and K34 in turn. Ability of the molecular clones to infect rabbits in vivo, is being tested by direct intramuscular DNA inoculation. HTLV-I antibodies were detected in the sera obtained from the rabbits injected with K30 plasmid by two months postinjection, HTLV-I specific sequences were detected by PCR from rabbit PBMC DNA and HTLV-I virus was isolated from PBMC culture indicating infection was achieved. Availability of such clones will facilitate functional studies of HTLV-I genes and gene products.